The present invention, in some embodiments thereof, relates to detection of chemicals and, more particularly, but not exclusively, to devices, systems and methods useful for detecting ultra-trace amounts of nitro-containing chemicals such as explosives in both liquid and gaseous phase.
An ‘explosive’ is a chemically-unstable molecule having a rapid rate of autodecomposition, with the accompanying evolution of large amounts of heat and gaseous products. There has been a great increase in the development of trace and ultra-trace explosive detection in the last decade, mainly due to the globalization of terrorist acts, and the reclamation of contaminated land previously used for military purposes.
In addition, the availability of raw materials for the preparation of explosives, together with the growing access to information on preparing these explosives, allows for almost anyone with sufficient will and internet access to prepare a bomb. The vast number of people passing through borders, public places, airports etc. poses a huge challenge for current day security screening technologies. The same challenge applies to homes and buildings security. The ultimate goal is of course to be able to rapidly and effectively screen every passing person, without the need to delay the traffic of people, and without human contact if possible.
Explosives, especially concealed ones, have a very low vapor pressure or ‘signature’ in the surrounding air. The effective vapor pressure of explosives can be reduced by terrorists by a factor of up to 1000, with the use of plastic packages. Detection methods for traces of explosives therefore continue to be plagued by the low volatility of many target analytes.
One of the most commonly-used high explosives over the last 100 years is 2,4,6-trinitrotoluene (TNT), which poses not only a direct security threat, but also great environmental concern due to soil and water contamination near production, storage and test sites.
Analytical procedures in use today for the trace detection of explosives typically involve collecting vapor samples and analyzing them with a sensitive method. Several methodologies have been reported for detecting TNT and other explosives. These are based on electrochemistry, ion-mobility spectrometry, gas chromatography, HPLC, photoluminescence, surface acoustic-wave devices, microcantilevers, fluorescent polymers, surface plasmon resonance, quartz crystal microbalance, immunosensors and other methods. In these existing methods, pre-concentration of air or liquid samples is usually required for a measurable signal to be recorded by the sensor. These procedures are timely, and delay the operation of a sensor. Although some reported methods are very sensitive and selective, most are rather expensive, time-consuming and require bulky equipment, tedious sample preparation and an expert operator. Furthermore, these systems cannot be miniaturized and automated or cannot perform high-throughput analysis.
Table 1 below presents data comparing TNT detection by various currently-employed methodologies.
TABLE 1DetectionDetection MethodLimitremote microelectrode electrochemical sensor in water50ppbluminescent oligo(tetraphenyl)silole nanoparticles in water20ppbquenching of fluorescence of polymer films in air10ppbelectrochemical detection by carbon nanotubes in water5ppbbiochip (on Au) via QCM or SPR in water1-10ppbelectrochemical detection using metallic NP-CNT composites in water1ppbadsorptive stripping by carbon nanotubes-modified GCE in water600pptelectrochemical detection by mesoporous SiO2-modified electrodes in414pptwateroligo(ethylene glycol)-based SPR in water80pptelectrochemical sensing by imprinted electropolymerized46pptbis-aniline-cross-linked AuNPs in waterSPR, fabricated dinitrophenylated-keyhole lympet hemocyanin (DNP-5pptKLH) protein conjugate (in water)indirect competitive immunoassay using SPR (in water)2pptSPR sensing by bis-aniline-cross-linked picric acid-imprinted1.2 × 10−3pptAu-nanoparticles composite in waterIMS (ion mobility spectroscopy) from air and water samples5-10pptSAW in water10pptconducting polymers in water20-40pptμ-Electron capture detector100pptAirport sniffers from air samples2000ppt
Specially-trained dogs can detect explosives with the use of their noses which are very sensitive to scents. These dogs are trained by expert handlers to identify the scents of several common explosive materials and notify the handler when they detect one of these scents. While being very effective, the usefulness of such dogs becomes easily degraded when a dog becomes tired or bored, thus limiting their range of application.
Semiconducting nanowires are known to be extremely sensitive to chemical species adsorbed on their surfaces. For a nanowire device, the binding of a charged analyte to the surface of the nanowire leads to a conductance change, or a change in current flowing through the wires. The 1D (one dimensional) nanoscale morphology and the extremely high surface-to-volume ratio make this conductance change to be much greater for nanowire-based sensors versus planar FETs (field-effect transistors), increasing the sensitivity to a point that single molecule detection is possible.
Nanowire-based field-effect transistors (NW-FETs) have therefore been recognized in the past decade as powerful potential new sensors for the detection of chemical and biological species. See, for example, Patolsky et al., Analytical Chemistry 78, 4260-4269 (2006); Stern et al., IEEE Transactions on Electron Devices 55, 3119-3130 (2008); Cui et al., Science 293, 1289-1292 (2001); Patolsky et al. Proceedings of the National Academy of Sciences of the United States of America 101, 14017-14022 (2004), all being incorporated by reference as if fully set forth herein.
Recently, extensive work has been carried out with the use of nanowire electrical devices for the simultaneous multiplexed detection of multiple biomolecular species of medical diagnostic relevance, such as DNA and proteins [Zheng et al., Nature Biotechnology 23, 1294-1301 (2005); Timko et al., Nano Lett. 9, 914-918 (2009); Li et al., Nano Lett. 4, 245-247 (2004)].
Generally, in a NW-FET configuration, the gate potential controls the channel conductance for a given source drain voltage (VSD), and modulation of the gate voltage (VGD) changes the measured source-drain current (ISD). For NW sensors operated as FETs, the sensing mechanism is the field-gating effect of charged molecules on the carrier conduction inside the NW. Compared to devices made of micro-sized materials or bulk materials, the enhanced sensitivity of nanodevices is closely related to the reduced dimensions and larger surface/volume ratio. Since most of the biological analyte molecules have intrinsic charges, binding on the nanowire surface can serve as a molecular gate on the semiconducting SiNW [Cui et al., 2001, supra].
U.S. Pat. No. 7,619,290, U.S. patent application having publication No. 2010/0022012, and corresponding applications, teach nanoscale devices composed of, inter alia, functionalized nanowires, which can be used as sensors.
Recently, Clavaguera et al. disclosed a method for sub-ppm detection of nerve agents using chemically functionalized silicon nanoribbon field-effect transistors [Clavaguera et al., Angew. Chem. Int. Ed. 2010, 49, 1-5]. McAlpine et al. [J. Am. Chem. Soc. 2008 Jul. 23; 130(29):9583-9] disclosed a scalable and parallel process for transferring hundreds of pre-aligned silicon nanowires onto plastic to yield highly ordered films for low-power sensor chips. The nanowires exhibit parts-per-billion sensitivity to NO2. SiO2 surface chemistries were used to construct a ‘nano-electronic nose’ library, which can distinguish acetone and hexane vapors via distributed responses [Nature Materials Vol. 6, 2007, pp. 379-384].
Additional background art includes U.S. patent application having Publication No. 2010/0325073.